Essentials of Anatomy and Physiology

ous system; the enteric nervous system can function
independently and promote normal peristalsis.
There are three sources of digestive secretions that
function within the small intestine: the liver, the pan-
creas, and the small intestine itself. We will return to
the small intestine after considering these other
organs.

LIVER

The liver(Fig. 16–6) consists of two large lobes, right
and left, and fills the upper right and center of the
abdominal cavity, just below the diaphragm. The
structural unit of the liver is the liver lobule, a
roughly hexagonal column of liver cells (hepatocytes).
Between adjacent lobules are branches of the hepatic
artery and portal vein. The capillaries of a lobule are
sinusoids, large and very permeable vessels between
the rows of liver cells. The sinusoids receive blood
from both the hepatic artery and portal vein, and it is
with this mixture of blood that the liver cells carry out
their functions. The hepatic artery brings oxygenated
blood, and the portal vein brings blood from the
digestive organs and spleen (see Fig. 13–7). Each lob-
ule has a central vein. The central veins of all the lob-
ules unite to form the hepatic veins, which take blood
out of the liver to the inferior vena cava.
The cells of the liver have many functions (which
are discussed in a later section), but their only diges-
tive function is the production of bile. Bile enters the
small bile ducts, called bile canaliculi, on the liver
cells, which unite to form larger ducts and finally
merge to form the hepatic duct, which takes bile out
of the liver (see Fig. 16–6). The hepatic duct unites
with the cystic duct of the gallbladder to form the
common bile duct, which takes bile to the duode-
num.
Bile is mostly water and has an excretory function
in that it carries bilirubin and excess cholesterol to the
intestines for elimination in feces. The digestive func-
tion of bile is accomplished by bile salts, which emul-
sifyfats in the small intestine. Emulsification means
that large fat globules are broken into smaller glob-
ules. This is mechanical, not chemical, digestion; the
fat is still fat but now has more surface area to facili-
tate chemical digestion.
Production of bile is stimulated by the hormone
secretin, which is produced by the duodenum when
food enters the small intestine. Table 16–2 summa-

The Digestive System 379

rizes the regulation of secretion of all digestive secre-

tions.

GALLBLADDER

The gallbladderis a sac about 3 to 4 inches (7.5 to 10

cm) long located on the undersurface of the right lobe

of the liver. Bile in the hepatic duct of the liver flows

through the cystic ductinto the gallbladder (see Fig.

16–6), which stores bile until it is needed in the small

intestine. The gallbladder also concentrates bile by

absorbing water (see Box 16–2: Gallstones).

When fatty foods enter the duodenum, the

enteroendocrine cells of the duodenal mucosa secrete

the hormone cholecystokinin. This hormone stimu-

lates contraction of the smooth muscle in the wall of

the gallbladder, which forces bile into the cystic duct,

then into the common bile duct, and on into the duo-

denum.

PANCREAS

The pancreasis located in the upper left abdominal

quadrant between the curve of the duodenum and the

spleen and is about 6 inches (15 cm) in length. The

endocrine functions of the pancreas were discussed in

Chapter 10, so only the exocrine functions will be

considered here. The exocrine glands of the pancreas

are called acini (singular: acinus) (Fig. 16–7). They

produce enzymes that are involved in the digestion of

all three types of complex food molecules.

The pancreatic enzyme amylasedigests starch to

maltose. You may recall that this is the “backup”

enzyme for salivary amylase, though pancreatic amy-

lase is responsible for most digestion of starch. Lipase

converts emulsified fats to fatty acids and glycerol.

The emulsifying or fat-separating action of bile salts

increases the surface area of fats so that lipase works

effectively. Trypsinogen is an inactive enzyme that is

changed to active trypsinin the duodenum. Trypsin

digests polypeptides to shorter chains of amino acids.

The pancreatic enzyme juice is carried by small

ducts that unite to form larger ducts, then finally the

main pancreatic duct. An accessory duct may also be

present. The main pancreatic duct emerges from the

medial side of the pancreas and joins the common bile

duct to the duodenum (see Fig. 16–7).

The pancreas also produces a bicarbonate juice

(containing sodium bicarbonate), which is alkaline.